1. Field of the Invention
The present invention relates to an electronic ballast for high-intensity discharge (hereinafter, referred to as "HID") lamp such as sodium vapor lamp, mercury lamp, metal halide lamp, and more particularly relates to the electronic ballast which employs a microprocessor to obtain a stable lighting control of the HID lamp by producing a driving signal in a transformed type of a White Noise Modulation method.
2. Description of the Prior Art
Up to now, the circuit configuration of the electronic ballasts for the HID lamp has been mostly an analog circuit or the circuit configurations based on a simple logic. These kinds of electronic ballasts have advantages compared with a digital control system in the case that the control is simple. But, in case of complicated circuit configurations and complicated control logic, the digital circuit system in which applied a microprocessor would rather be more advantageous in terms of a circuit configuration and a cost.
In U.S. Pat. No. 5,428,268, an electronic ballast for the HID lamp has been disclosed. In that patent, anyone can see that a ballast circuit has a very complicated configuration since the ballast circuit is made of an analog circuit and further a simple logic circuit.
As a method of driving the HID lamp, there have been three typical driving methods: a method using a frequency of several tens or hundreds of Khz for lighting the HID lamp, a method using a modulation of a carrier square wave of low frequency by a wave of a high frequency and a method using a frequency-shift. The modulation method has a disadvantage in dimming operation, and the frequency shift method has a defect in stabilizing an arc of the HID lamp because conventional HID lamps have different characteristics according to manufacturers. Especially, configuring the ballast of the metal halide lamp with an electronic type may cause an unstable flame fluctuation or acoustic-resonance, therefore an arc tube is broken at the worst.
A document entitled "White-Noise Modulation of High-Frequency High-Intensity Discharge Lamp Ballasts" was disclosed by Laszlo Laskai, Prasad N. Enjeti and Ira J. Pitel in IEEE Transactions Industry Application Vol. 14, No. 3, May/June 1998. In the document, in order to avoid acoustic-resonance-related instabilities in metal halide lamps operated with a high-frequency electronic ballast, a new method using angle-modulation was proposed to randomize a switching frequency of an inverter and to limit a lamp power spectrum below a threshold value. But, even this method still have a defect of audible noise in high-frequency, the higher the number of wattage is, the louder the audible noise is.
There is a method of using a half-bridge inverter circuit. This method also has a problem that when a ballast has a high power, a phenomenon of a wave shift happens during a switching operation of the half-bridge inverter. And, the wave shift causes an imbalance of heat generations between an upper switching section and a lower switching section, so that the life of ballast becomes short.